Autopage took money from my account AGAIN!!

Autopage has been harrassing me with sms' for an amount of R800 outstanding because they were not debiting my account. I paid R1000 to them at the end of March covering the R800 and the month of April, paying that in advance. Autopage has gone and debited R800 from my account again using an agency called NUDEBT. I've been receiving numerous sms threating to have me blacklisted. If my name is taken to credit bereau , I WILL SUE THEM. I owe them nothing!!! I can never get through to their accounts department. Their consultants never know what to do when I call in. I always have to call them but never get answers. This is not the first time they have done this. I am TIRED of the constant headaches that autopage gives me. I even made payment arrangements with them before I decided to pay the full amount and prepay the next month. Clearly that did not stand anymore. I am so sick of autopage. I didn't even chose to be with them.